The Radio Continuum Halos of the Edge-on Galaxies NGC 891 and NGC 4565

Abstract

The halos of spiral galaxies are complex entities consisting of multiple matter components, generally including neutral and ionized gas phases, cosmic rays (CRs), dust, as well as extraplanar stars. The detection of extraplanar synchrotron radiation from cosmic-ray electrons (CREs) furthermore shows that halos, like galactic disks, are magnetized. Halos play an integral part in the circulation of matter in galaxies, as their gaseous content is closely connected to processes occurring in galactic disks, such as star formation. At the same time, halos represent an interface between the disk-internal interstellar medium (ISM) and the intergalactic medium (IGM). However, a coherent picture of how halos are connected to disks and to their environments, as well as of their role in galaxy evolution, has yet to be observationally established. Even so-called normal spiral galaxies are diverse in their physical properties, and studying their extraplanar matter distribution is a key contribution to our understanding of this diversity. <br /> Edge-on galaxies are ideal laboratories to investigate the connection of halos to disks and to the IGM, since only in an edge-on view the properties of halos can be directly studied as a function of distance perpendicular to the disk. Two prominent examples of edge-on galaxies, NGC 891 and NGC 4565, which differ greatly in their star formation rates (SFRs), are the subject of this thesis. The main focus of the studies presented here is on the propagation of cosmic rays within the halos of the two galaxies and on their extraplanar magnetic field structure. Cosmic rays and galactic magnetic fields are traced by synchrotron radiation of the CREs, which is emitted in the radio continuum frequency regime. This thesis is primarily based on observations of NGC 891 and NGC 4565 in the framework of Continuum HAlos of Nearby Galaxies – an EVLA Survey (CHANG-ES). This full-polarization survey makes use of the novel broad-band technology of the recently upgraded Karl G. Jansky Very Large Array (JVLA) to study a sample of 35 edge-on galaxies at radio frequencies of 1.5 GHz (L-band) and 6 GHz (C-band), at various spatial resolutions.<br /> We investigate the spectral index distribution in NGC 891 and NGC 4565 at an angular resolution of 1200. By comparing our results for the two galaxies, we find that spectral indices are consistently steeper in NGC 4565, in the halo as well as in the disk. In this galaxy, the steepening of the local spectra between the two observing frequencies is consistent with single-shot CR injection models rather than continuous CR injection. We conclude from this that NGC 4565 must locally feature very low CR injection rates compared to those in NGC 891, which would be consistent with its low SFR and would serve as a plausible explanation for its low-level disk and halo emission.<br /> By fitting two-component (disk and halo) exponential profiles to the vertical emission distribution, we determine vertical scale heights at several positions along the major axis in both galaxies. On average, we obtain roughly similar scale heights for the halo components of the two galaxies, as already known from previous studies at lower spatial resolutions. However, while in NGC 891 the scale heights are basically identical at 1.5 and 6 GHz, for NGC 4565 we obtain slightly different results at the two frequencies. These findings tentatively indicate that CR transport is dominated by convection in NGC 891 and at least to a significant degree happening by diffusion in NGC 4565.<br /> The average equipartition magnetic field strength in the disk is 10 μG for NGC 891 and 5 μG for NGC 4565. For the case of NGC 891, we model the CR transport in the halo by solving the vertical diffusion-loss equation. Along with the vertical emission profiles at 1.5 and 6 GHz, we simultaneously model the vertical decrease of the total magnetic field strength. We find that our data are fitted well by a purely convective CR propagation model, with convection speeds of likely more than 300 km s^-1, indicating that CRs are able to leave the gravitational potential of the galaxy and escape as a galactic wind. Moreover, we obtain a halo magnetic field scale height ≈40% lower than the expected equipartition value, which shows that the assumption of energy equipartition between CRs and magnetic fields is not valid in the halo of NGC 891.<br /> To study the magnetic field structure in NGC 891 and NGC 4565, we analyze our polarization data by applying the technique of rotation measure (RM) synthesis. The insights gained from the determined RM distributions are rather limited, as the L-band data are severely affected by depolarization through differential Faraday rotation. Nonetheless, we are able to show by these measurements that NCG4565 clearly possesses a non-plane-parallel halo field, which is probably X-shaped (like in NGC 891), as predicted by galactic dynamo models that include disk-halo outflows.<br /> Our high-resolution (300) total intensity maps of NGC 4565 reveal a ring-shaped structure of the inner spiral arms that was previously only seen in the dust and molecular gas components. Given the high inclination of 86°, the fact that the front side of this ring-shaped pattern is resolved from its back side implies that these inner spiral arms must be comparatively thin, either in the vertical or coplanar direction.